With the increase of the technological development and demand for a mobile device, demand for a battery as an energy source rapidly increases, and accordingly, many researches of the battery meeting various requests are conducted. Representatively, in view of a battery shape, a prismatic secondary battery and a pouch type secondary battery that can be applied to products such as a mobile phone with a thin thickness shows high demand, and in view of a battery material, a lithium secondary battery such as lithium ion battery and lithium ion polymer battery having advantages such as high energy density, discharging voltage, and output stability shows high demand.
Further, the secondary battery may be classified according to a structure of a positive electrode/separator/negative electrode structure of an electrode assembly. There may be, for example, a jelly-roll type electrode assembly having a structure in which long sheets of positive electrodes and negative electrodes are wound while a separator is interposed therebetween, a stack type electrode assembly having a structure in which a plurality of the positive electrodes and the negative electrodes, cut by a certain size unit, are sequentially stacked while the separator is interposed therebetween, and a stack/folding type electrode assembly having a structure in which bi-cells or monocells stacked with certain units of the positive electrodes and the negative electrodes are wound with a separation film and so on while a separator is interposed therebetween.
Recently, a pouch type battery having a structure in which the stack type electrode assembly is placed within an aluminum laminate sheet of a pouch type battery case gathers much interest with reasons of low manufacturing cost, small weight, and easy shape modification, and further, utilization thereof gradually increases.
FIG. 1 briefly illustrates a process of manufacturing the monocell to prepare the electrode assembly of a related stack type battery. A minimum unit monocell may generally include separator/first electrode/separator/second electrode. As illustrated in FIG. 1, a unit monocell may be manufactured by sequentially stacking separator 11/first electrode 12/separator 11/second electrode 13, interlayer-bonding each layer by heating and pressing according to a roll lamination process, and cutting the tightly contacted layers properly with cutters C1, C2, C3. A related bi-cell manufacturing process may be also manufactured in a similar format to the above monocell manufacturing process even though there is difference in a stacking order of the electrodes and the separators. However, the monocell and/or the bi-cell manufactured through the roll lamination process has a disadvantage in which it may be bent with the heat and the pressure applied while passing through a roller and may have a curved shape instead of a flat plate shape. Further, as illustrated in FIG. 1, the separators and the electrodes may be stored in a wound state until it is unwound and provided to a roll lamination for stacking. Such curvature phenomenon may more prominently appear when the modification applied during wound state is not sufficiently removed by the roll lamination. Because the stack type battery may be used in manufacturing a prismatic battery having a flat plate shape, an additional process to modify such a curved shape monocell and/or bi-cell into a flat plate shape may be needed.
FIG. 2 briefly illustrates a related stack type battery manufacturing process. First, the electrode assembly 100 may be manufactured by piling and stacking the bent unit cells 10 (monocell and/or bi-cell) so that directions of curvature are same as each other, followed by taping 20 and fixing. The electrode assembly may maintain a curve shape. Thereafter, the electrode assembly may be modified into a flat plate shape by simultaneously pressing upper/lower portions of the electrode assembly with the curved flat plate jig J. However, because directions of curvature are same in the stacked unit cells, the modifying stresses remained on a curved portion may be strengthened by each other between the unit cells, and accordingly, there is a problem that removing the curvature from the electrode assembly may not be easy.